Fiber and whisker reinforced composites and method for making the same

ABSTRACT

A composite material (20) comprises a matrix layer (21) having a plurality of interspersed reinforcing whiskers (23) and a plurality of continuous reinforcing fibers (25) embedded within the matrix layer (21). The preferred embodiment includes a matrix layer (21) which may be a ceramic, intermetallic or metallic material having interspersed reinforcing whiskers (23) upon which a second layer of the matrix (24) having embedded continuous reinforcing fibers (25) is placed, and a third layer (22) of the matrix material having the interspersed reinforcing whiskers (23) on the second layer (24). The composite exhibits improved fracture toughness due to the crack deflection ability of whiskers (23) and crack bridging and fiber pull out due to continuous fibers (25) and minimizes creep associated with known ceramic and intermetallic composites.

RELATED APPLICATIONS

This application is a division of application Ser. No. 07/726,981, filedJul. 8, 1991 and entitled "FIBER AND WHISKER REINFORCED COMPOSITES ANDMETHOD FOR MAKING THE SAME".

TECHNICAL FIELD OF THE INVENTION

Generally, the present invention relates to composite materials andmethods for making composite materials, and more particularly to acombined continuous fiber and whisker reinforced composite materialhaving improved material properties.

BACKGROUND OF THE INVENTION

For a variety of applications, equipment and component designers arefinding uses for ceramic, intermetallic, and metallic composites. Onereason for this trend is that the ability of composite materials towithstand high temperature stresses as structural elements is greatlydesired. However, the ceramic and intermetallic composites typically mayfracture under the strain of use due to brittleness and tend to creep athigh temperature. As a result, these materials are not entirely suitablefor structural components in numerous applications.

Some known attempts to overcome the brittleness and creep problems usefibers or whiskers to reinforce the ceramic and intermetalliccomposites. For example, in one effort a MoSi₂ matrix was reinforcedwith 20 volume percent SiC whiskers to achieve a 54% increase in thefracture toughness and a 100% increase in the flexural strength of thematerial. In this effort, the fracture toughness of 8.2 MPa.m^(1/2) wasobtained. Although this represents a significant improvement in thematerial properties of the composite material, the fracture toughnessstill falls short of the acceptable regime for structural components inmost applications. In many structural component applications, aconsistent fracture toughness level of 12-15 MPa.m^(1/2), is desirable.As a result, significant room for improvement exists in the fracturetoughness of composite materials. Additionally, known applications ofwhisker impregnated composite materials still exhibit considerable creepat high temperatures under load.

Thus there is a need for a composite material that does not exhibit thebrittleness of known ceramic, intermetallic, and metallic composites.

There is a need for a composite for use in structural components thatpossesses improved fracture toughness.

There is yet the need for a ceramic and intermetallic composite thatadvantageously uses whiskers for increased flexural strength, as well asfurther providing increased fracture toughness beyond known levels.

There is furthermore the need for an improved composite material thatavoids the high temperature creep phenomenon of known composites.

SUMMARY OF THE INVENTION

The present invention, accordingly, overcomes the problems andlimitations associated with known ceramic and intermetallic compositesto provide a composite having improved strength, fracture toughness, andthat may be used at high temperatures with minimal creep.

According to one aspect of the invention, there is provided a compositecomprising a layer of ceramic or intermetallic matrix having a pluralityof interspersed reinforcing whiskers in combination with a plurality ofembedded continuous reinforcing fibers.

According to another aspect of the invention, there is provided a methodfor making a composite comprising a ceramic or intermetallic matrix. Themethod comprises the steps of mixing reinforcing whiskers and a ceramicor intermetallic powder with a binder material at a temperature ofapproximately 100° C. The mixture is then made into sheets which areheated so that the binder vaporizes to yield thin sheets of compositewith interspersed reinforcing whiskers. Next, in the preferredembodiment, silicon carbide fibers are rolled from a cylinder or drumand laid on a predetermined number of the thin composite sheets to formthe desired composition. In the preferred embodiment, the composite hasa layer of molybdenum disilicide reinforced with silicon carbidewhiskers followed by a molybdenum disilicide layer having continuoussilicon carbide fibers. The resulting composite exhibits improvedproperties for very high temperature applications.

A technical advantage of the present invention is that the compositematerial strengthens the matrix at high temperatures as a result of thepresence of the whiskers and fibers. This is because the fibers act as aload bearing member in high temperature applications.

Another technical advantage of the present invention is that at low andhigh temperatures the whiskers toughen the matrix by crack deflectionand the continuous fibers further toughen the matrix by both crackbridging and fiber pull out within the composite.

Yet another technical advantage of the present invention is that in hightemperature applications the composite minimizes the creep phenomenatypical of known ceramic or intermetallic composites.

Still another technical advantage of the present invention is that thesilicon carbide whiskers reduce substantially the silicon richlow-temperature grain boundary phase that forms during hot pressing of apure MoSi₂ matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its modes of use and advantages are best understood byreference to the following description of illustrative embodiments readin conjunction with the accompanying drawings, wherein:

FIG. 1 is a perspective view of a composite typical of prior art havingwhiskers distributed therethrough;

FIG. 2 is a perspective view of a typical continuous fiber composite ofthe prior art possessing fiber reinforcement;

FIG. 3 provides a perspective view of the preferred embodiment; and

FIG. 4 shows a perspective view of an alternative embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention is best understood byreferring to the FIGUREs, wherein like numerals are used for like andcorresponding parts of the various components.

FIGS. 1 and 2 provide perspective views of prior art composites 10. Theintermetallic or ceramic matrix 11 of FIG. 1 possesses whiskers 12 thatdistribute substantially uniformly within the ceramic reinforcingmaterial. On the other hand, composite 10 of FIG. 2, shows matrix 13which comprises continuous strands 14 of ceramic fiber. Both of theseconfigurations, if used alone, do not satisfactorily exhibit sufficientfracture toughness for use as structural components, nor do theysubstantially reduce the undesirable creep phenomena of the matrix athigh temperatures. In the case of molybdenum disilicide reinforced withapproximately 20 volume percent of silicon carbide, a fracture toughnessnearly 8.2 MPa.m^(1/2) has been obtained. This level falls short of anacceptable requirement for structural components, particularly for thosecomponents used in the aerospace industry. For these types ofapplications, a fracture toughness level of 12-15 MPa.m^(1/2) isnecessary. FIGS. 3 and 4, on the other hand, illustrate a preferred andan alternative embodiment of the present invention which overcome thelimitations inherent in known ceramic and intermetallic composites.

With reference to FIG. 3, there is shown a three-layer composite 20 inwhich upper layer 21 and lower layer 22 are reinforced with whiskers 23and intermediate layer 24 is reinforced with fibers 25. FIG. 4 providesan alternative embodiment showing a different layer configuration fromthat of FIG. 3. The orientation of layers 31 of composite 30 havingfibers 32 provides perpendicular layers of fiber mats that haveadditional strength in the direction perpendicular to the direction offibers in FIG. 3. Obviously, instead of being perpendicular the angle ofcontinuous fibers 32 may vary.

An important aspect of the preferred embodiment is the combination ofcomposite layers with different forms of ceramic (e.g., silicon nitride,silicon oxide, aluminum oxide, or zirconium oxide), intermetallic (e.g.,molybdenum disilicide, rhenium silicide, tungsten silicide, nickelaluminide, titanium aluminide, iron aluminide, or niobium aluminide), ormetal (e.g., W, Mo, Ta and other similar metals) reinforcement. It isalso possible to form ceramic layers in the present invention thatutilize pre-ceramic polymers that become ceramic during processing.Moreover, mixtures of ceramic, intermetallic, pre-ceramic or metals maybe used as the matrix layers for the present invention. The preferredembodiment, however, uses ceramic reinforcement for the composite.

Although the preferred embodiment uses at least one layer that ceramicwhiskers reinforce and at least another layer that ceramic fibersreinforce, the combined ceramic whisker and ceramic fiber reinforcementmay take place in one layer. It will be evident, however, that thenumber of layers of differentially reinforced elements of the compositemay vary widely depending on the composite's desired end use.

The preferred embodiment uses a combination of whisker reinforcement andfiber reinforcement within the composite layer. As such, one layer isreinforced by whiskers and another layer is reinforced with fibers.Irrespective of the number of layers used, which may vary according tothe particular use of the composite of the present invention, there arepreferred materials for both the whiskers and the fibers. For thewhiskers, ceramic materials such as silicon nitride or silicon carbideare preferable. However, any known and available materials such assilicon carbide, alumina, carbon, titanium diboride, tungsten,silicon-nitride, niobium, or other similar materials may also serve asreinforcing whiskers. Again, the particular material for the whiskersdepends in large part by the ultimate use of the composite and thecompatibility of the whiskers to the matrix. For the reinforcing fibers,a ceramic or other material similar to the material of the whiskers isappropriate. A major difference, however, is that the reinforcing fiberstake a long continuous fiber form as opposed to the short discontinuousform of the reinforcing whiskers.

Depending on the desired properties of the final composite, theproportions of whiskers and fibers may vary. For example, the degree ofwhisker reinforcement in any layer may vary from one percent to fiftypercent by volume of the matrix. Concerning the fibers which arecontinuous throughout the matrix, they may vary from one percent totwenty percent by volume in the composite.

The composites of the preferred embodiment provide a strengthened matrixin which the continuous fibers act as load bearing members and thereinforcing whiskers reduce the propensity of the composite to exhibitcreep. Additionally, at low temperatures, the reinforcing whiskersmaximize crack deflection while the continuous fibers enhance crackbridging and fiber pull-out. Whiskers in the preferred embodiment alsoreduce substantially the silicon-rich low-temperature grain boundaryphase that forms during high temperature processing of certainintermetallics such as molybdenum disilicide.

Having described the composition of the preferred embodiment, it is nowappropriate to describe its preferred manufacturing process. Thepreferred process of manufacturing the composites requires formation ofindividual reinforced layers by powder injection molding or tape castingfollowed by combining the layers to form a composite. To explain themanufacturing process of the preferred embodiment, consider acomposition of molybdenum disilicide powder as the matrix material andsilicon carbide as the reinforcing material for both the whiskers andthe continuous fibers.

Formation of the whisker reinforced layer occurs by mixing themolybdenum disilicide powder with the silicon carbide whiskers and abinder which may be a polymer, a combination of polymers, or acombination of polymers with a wax or oil. The polymer or combination ofpolymers in wax or oil should be solid at room temperature and molten atlow temperatures of approximately 100° C. Suitable polymers for thisstep of the process are polystyrene and polypropylene, while suitablewax may be carnauba wax.

At this stage, also, pre-ceramic polymers that are fluid during thesheet forming stage and that can be converted to ceramic during asubsequent debinding stage may also be used. In any event, approximatelyequal volumes of binder and mixture of reinforcing whiskers and matrixmaterial are used at this stage. Mixing of binder and the whisker-matrixmixture is performed at elevated temperatures and preferably in a vacuumatmosphere. The resultant mixture or "feed stock" is then formed intosheets of desired thickness by any conventional means such as injectionmolding, extrusion, tape casting or compression molding and, if desired,the whiskers may be aligned during the molding in the direction of flow.

The sheets of the feed stock are then heated to a temperature above themelting point of the binder. Typically, this can be accomplished byheating the feed stock to about 800° C. This causes the binder todisintegrate and remove the volatile products with flowing gases overthe sheets.

Sheets of silicon-carbide fiber-reinforced matrix are formed bycontinuous strands of the silicon carbide fibers, held together by apolymeric glue and held on a drum to form a sheet. The polymer-boundsheets of fibers are used as one of the layers used to form thecomposite. The composite is formed by placing alternate layers of thefiber and whisker-reinforced materials on top of each other to form thedesired height and shape of the composite. The shape is then hot pressedat a temperature sufficient to sinter the layers, usually about1500°-1800° C. to form the actual composite. The number of layers andorientation of the layers may vary, of course, depending on the desiredproperties of the final composite.

In summary, the above description details a new composite possessingimproved material properties at both high and low temperatures. Thecomposite uses both reinforcing whiskers and continuous fibers in aunique architecture to improve load bearing characteristics over knowncomposites. Moreover, described methods for manufacturing the compositemake the composite easily adaptable to a wide variety of industrialapplications.

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationsmay be made herein without departing from the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A method of forming a composite materialpossessing a matrix layer having a plurality of interspersed reinforcingwhiskers and a plurality of continuous reinforcing fibers, comprisingthe steps of:forming a mixture of matrix layer material, the whiskers,and a binding material; forming sheets of said mixture; heating saidsheets to burn out said binder; associating a plurality of continuousfibers in a predetermined number of said plurality of sheets; forming asingle multi-layered form having alternating layers of said sheetswherein a first of said alternating layers has no associated continuousfibers followed by a sheet having associated continuous fibers; andsintering said single multi-layered form to produce a single compositematerial possessing both interspersed whiskers and continuousreinforcing fibers therewithin.
 2. The method of claim 1, wherein saidmulti-layer forming step further comprises the step of stacking saidsheets containing continuous fibers to produce alternating layers suchthat a predetermined number of said continuous fibers are perpendicularto one another.
 3. The method of claim 1, wherein said sheet formingstep comprises injection molding said mixture.
 4. The method of claim 1,wherein said sheet forming step comprises extruding said mixture.
 5. Themethod of claim 1, wherein said sheet forming step comprises tapecasting said mixture.
 6. The method of claim 1, wherein said sheetforming step comprises the step of compressing said mixture.
 7. Themethod of claim 1, wherein said fiber associating step comprises thestep of embedding said continuous fibers in a predetermined number ofsaid sheets.
 8. The method of claim 1, wherein said continuous fiberassociating step comprises the step of associating a polymer-bound sheetof said continuous fibers with said predetermined number of sheets. 9.The method of claim 1, wherein said sintering step comprises the step ofheating said multiple layer form to a temperature of between 1500° and1800° C.
 10. The method of claim 1, wherein said multi-layer formingstep comprises the step of laying first a sheet comprising said matrixlayer having interspersed reinforcing whiskers, then laying on saidfirst layer a second layer comprising said continuous reinforcingfibers, and thenlaying on said second layer opposite said first layer athird sheet of said matrix layer comprising said interspersedreinforcing whiskers.
 11. The method of claim 10, wherein saidmulti-layer forming step further comprises the steps of laying on saidthird sheet a fourth layer comprising said reinforcing whiskers;layingon said fourth layer a fifth layer comprising continuous reinforcingfibers, said reinforcing fibers oriented perpendicular to saidreinforcing fibers in said second layer; and laying on said fifth layera sixth layer comprising said interspersed reinforcing whiskers.